Although tabular grains had been observed in silver bromide and bromoiodide photographic emulsions dating from the earliest observations of magnified grains and grain replicas, it was not until the early 1980's that photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of blue and minus blue imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, were realized to be attainable from silver bromide and bromoiodide emulsions in which the majority of the total grain population based on grain projected area is accounted for by tabular grains satisfying the mean tabularity relationship: EQU D/t.sup.2 &gt;25
where
D is the equivalent circular diameter (ECD) in micrometers (.mu.m) of the tabular grains and
t is the thickness in .mu.m of the tabular grains. Once photographic advantages were demonstrated with tabular grain silver bromide and bromoiodide emulsions techniques were devised to prepare tabular grains containing silver chloride alone or in combination with other silver halides. Subsequent investigators have extended the definition of tabular grain emulsions to those in which the mean aspect ratio (D:t) of grains having parallel crystal faces is as low as 2:1.
Notwithstanding the many established advantages of tabular grain silver bromide and bromoiodide emulsions, the art has observed that these emulsions tend toward more disperse grain populations than can be achieved in the preparation of regular, untwinned grain populations--e.g., cubes, octahedra and cubo-octahedral grains. This has been a concern, since reducing grain dispersity is a fundamental approach to reducing the imaging variance of the grains, and this in practical terms can be translated into more nearly uniform grain responses and higher mean grain efficiencies in imaging.
Tsaur et al U.S. Pat. Nos. 5,147,771; 5,147,772 and 5,147,773 and U.S. Ser. No. 700,019, filed May 14, 1991, commonly assigned and now U.S. Pat. No. 5,171,659 titled PROCESS OF PREPARING A REDUCED DISPERSITY TABULAR GRAIN EMULSION, (hereinafter collectively referred to as Tsaur et al) has provided a solution to the problem of elevated grain dispersities in tabular grain emulsions. Tsaur et al employs a post nucleation solvent ripening process for preparing tabular grain emulsions. That is, at a point in the precipitation process in which the grains contain the parallel twin planes necessary for tabularity a silver halide solvent is introduced to ripen out a portion of the grains. This narrows the dispersity of the grain population and reduces the dispersity of the final tabular grain emulsion produced. The post nucleation solvent ripening processes of Tsaur et al further reduce total grain dispersity in precipitating tabular grain emulsions by introducing a selected polyalkylene oxide block copolymer surfactant containing both hydrophilic and lipophilic block units into the dispersing medium at the outset of tabular grain formation.
Tsaur et al has been able to produce tabular grain emulsions in which the grain size dispersity of the total grain population is quite low. A technique for quantifying grain dispersity that has been applied to both nontabular and tabular grain emulsions is to obtain a statistically significant sampling of the individual grain projected areas, calculate the corresponding ECD of each grain, determine the standard deviation of the grain ECDs, divide the standard deviation of the grain population by the mean ECD of the grains sampled and multiply by 100 to obtain the coefficient of variation (COV) of the grain population as a percentage. The Tsaur et al precipitation processes are generally applicable to producing tabular grain emulsions having a relatively low dispersity of the total grain population (COV&lt;30 percent). In most instances the precipitation processes of Tsaur et al produce tabular grain emulsions with a total grain population COV of less than 20 percent and, under specifically selected conditions, with a total grain population COV of less than 10 percent, an extremely low dispersity level for tabular or nontabular grain emulsions.
Although Tsaur et al has effectively solved the long standing problem of grain dispersity in tabular grain emulsions, the precipitation processes of Tsaur et al have presented the disadvantage that the presence of a polyalkylene oxide block copolymer surfactant in the dispersing medium at the outset of tabular grain formation slows the growth of the tabular grains. In other words, for a given elapsed period of precipitation a lower average tabular grain ECD is realized using any one of the Tsaur et al processes as compared to a comparable process not employing the polyalkylene oxide block copolymer surfactant. The elapsed time to reach a selected average tabular grain ECD, particularly where moderate and higher(&gt;2 .mu.m) tabular grain ECDs are contemplated, is a matter of importance in the manufacture of photographic materials.